Ionized physical vapor deposition such as is used in the manufacture of semiconductors is carried out by ionizing metal or other coating material in a high-density plasma then directing the ionized particles of coating material across the potential drop of a plasma sheath onto the substrate. Sources for creating such plasmas are either capacitively coupled or inductively coupled. Capacitively coupled sources energize processing gas within a vacuum processing chamber by coupling energy via an RF electrical field from electrodes. Inductively coupled plasma sources couple energy via an RF magnetic field from an antenna.
Capacitive sources are generally regarded as inferior to inductive sources because they produce a lower plasma density and large negative self-bias at the electrode. These characteristics of typical capacitive sources usually make them inappropriate for iPVD applications. Low plasma density is fundamentally related to the large RF voltage of capacitively coupled sources. Plasma density is a result of a balance between the energy input into the plasma by the RF and energy losses due to atomic processes and, more importantly, the kinetic energy or the ions falling through the plasma sheath and leaving the plasma. The large sheaths generated at the RF electrodes, which is the characteristic of a large potential difference between the plasma and the electrode, serve as sinks of plasma energy, and lead to the reduction in plasma density. In iPVD applications, low plasma density reduces the metal ion fraction. In addition, the large sheath voltage at the RF electrode leads to the sputtering of the electrode material into the plasma, reducing its lifetime.
ICP sources tend to be complex, particularly where they are designed to optimize deposition uniformity. Antennas and baffles must be designed using sophisticated methods.
Experimental work is reported on various plasma sources for certain applications. Furuya & Hirono examined the effects of magnetic field strength on the sputtering rate and bias voltage of an RF magnetron. They observed a reduction of the self-bias voltage, and an increase in plasma density as the field strength was increased. In addition, they observed a decrease in the sputtering rate as the magnetic field increased above 400 Gauss. In their experiment, a 4 in (10 cm) diameter CrCo target was used and the experiment was performed at 10 mTorr (1.33 Pa) and 200 W of RF power. [Furuya A & Hirono S, 1990, J. Appl. Phys., 68(1), 304 10.]
Further, I et al. (1984) examined the effects of magnetic field strength and pressure on the etch rate in a modified MRC RIE-51 diode etching system. With the magnetic field varied from 60 to 240 G, the bias voltage on the RF electrode dropped from 500 to 50 V. [I L, Hinson D C, Class W H & Sandstrom R L 1984 Appl. Phys. Lett. 44(2), 185.]
Magnetic field effects in a capacitive source with a localized magnetic field was explored by Wickramanayaka & Nakagawa (1998), for an embedded array of magnets in the top RF electrode of a dual-frequency capacitively coupled source for large area processing. The magnets are oriented in a checker-board fashion, leading to a rapid decrease of the magnetic field away from the electrode surface. The addition of magnets leads to a three-fold increase in plasma density, and a corresponding decrease in the self-bias voltage was observed. [Wickramanayaka S & Nakagawa Y 1998 Jpn. J. Appl. Phys. 37(11, Pt. 1), 6193.]
Further, Kaufman & Robinson (1993) proposed a broad beam ion source for space propulsion and industrial applications. [U.S. Pat. No. 5,274,306.]
Accordingly, there is a need for a simple high-density plasma source having low risk of producing discharges or contaminating particles.